Interface CompletionService<V>

A service that decouples the production of new asynchronous tasks
from the consumption of the results of completed tasks. Producers
submit tasks for execution. Consumers take
completed tasks and process their results in the order they
complete. A CompletionService can for example be used to
manage asynchronous IO, in which tasks that perform reads are
submitted in one part of a program or system, and then acted upon
in a different part of the program when the reads complete,
possibly in a different order than they were requested.

Typically, a CompletionService relies on a separate
Executor to actually execute the tasks, in which case the
CompletionService only manages an internal completion
queue. The ExecutorCompletionService class provides an
implementation of this approach.

Memory consistency effects: Actions in a thread prior to
submitting a task to a CompletionServicehappen-before
actions taken by that task, which in turn happen-before
actions following a successful return from the corresponding take().